The present invention relates generally to communication devices, and in particular to a Radio Frequency amplifier portion of a communication device.
The frequency spectrum that is shared among radio communication devices is limited. Thus the ability of a transmitter to transmit as much information as possible in an allocated frequency spectrum or channel without interfering with other communication devices in off channels is of great importance. To transmit as much information as possible in the allocated channel, digital communication systems typically modulate both the amplitude and phase of a radio frequency (RF) carrier. The amplitude modulation allows more information to be encoded on the carrier in a given channel than if only the phase was modulated. However, the amplitude modulation puts additional requirements on the transmitter that would not exist if only the phase of the RF carrier was modulated.
These additional requirements are due to the inherent nonlinear effects resulting from the amplification of an amplitude modulated signal by an RF power amplifier in a linear transmitter in the communication device. The non-linear effects are due to the amplitude compression characteristics (AM/AM) and the phase distortion (AM/PM) characteristics of the RF power amplifier when it is driven over a range of amplitudes. If these non-linear characteristics are not compensated they will cause spreading of the spectrum into the off channels and thus interfere with communication devices using off channels.
One method of maintaining a high degree of transmitter linearity is to operate a transmitter, that is, a radio frequency (RF) amplifying element in the transmitter, at a highly linear bias such as a class A bias and to back off transmitter output power so as not to drive the RF amplifying element into amplitude compression. However, a drawback to class A bias operation is low efficiency. Efficiency is a measure of the level of conversion of input RF power and input DC power to output RF power. Class A bias amplifiers typically have efficiencies of well under 50% while class C (non-linear) bias amplifiers can approach 85% efficiency. The result of lower efficiency operation is greater power consumption to produce a desired level of output power, more limited RF output power for a given DC power source, and more complex thermal issues since an implication of lower efficiency is the dissipation of a greater percentage of the power consumed in the form of heat. These issues are critical when amplifier operation is constrained by battery life or when heat dissipation is constrained by transmitter size.
A number of prior art signal processing techniques have been developed to allow a transmitter to operate using a non-linear bias level and to compensate for the non-linearities introduced as a result of the non-linear bias level. Among these techniques are predistortion, adaptive predistortion, feedforward correction loops and feedback correction loops. Predistortion and adaptive predistortion seek to inject a predistorted signal into an input signal""s path prior to amplification. The predistorted signal includes components equal and opposite to the distortion introduced by the power amplifier and is designed to cancel the distortion introduced to the input signal by the power amplifier. However, the application of a predistorted injection signal is limited due to the difficulty of characterizing a power amplifier and correcting for the power amplifier""s characteristics with a predetermined function. Feedback and feedforward are real time correction techniques and therefore do not require characterization of the power amplifier. However, feedforward correction includes costly error amplifiers, which may also introduce distortion into the feedforward path, and additional system expense and complexity such as carefully matched delays between the input signal forward path and the feedforward loop. Therefore, a technique commonly used to improve transmitter linearity is negative feedback correction. Typically, in negative feedback correction, a feedback signal from a Cartesian feedback loop in the transmitter is combined with an input signal sourced to the transmitter by a signal source to correct distortion introduced to the input signal by the transmitter""s amplification circuitry.
Many Cartesian feedback systems require an initialization period, and subsequent periodic training periods, to train the feedback loop. That is, when a transmitter using Cartesian feedback begins transmitting, the phase shift of the feedback path in the feedback loop must be adjusted, or trained, so that the feedback signal is properly out-of-phase with the input signal and will cancel out the distortion introduced to the input signal by the amplification circuitry. During such training periods, the transmitter runs open loop because of the potential for transmitter instability prior to the alignment of the phase shift of the feedback path. However, running open loop eliminates the negative feedback correction provided by the feedback path, allowing for the uncompensated transmission by the transmitter of distortions introduced to the input signal and resulting in spectral spreading.
Furthermore, in amplitude modulation schemes such as quadrature amplitude modulation (QAM), the input signal is usually a baseband quadrature signal that includes an in-phase (I) component and a quadrature (Q) component. Quadrature signals can be represented in a complex (I/Q) plane as a vector, which vector includes an in-phase (I) component and a quadrature (Q) component. A measure of linearity is how closely the in-phase and quadrature components of a transmitted signal, or vector, matches the in-phase and quadrature components of the input signal, another vector, wherein the difference between the two vectors constitutes an error vector. During the open loop training period of a transmitter that uses a Cartesian feedback loop, it is possible to produce significant error vectors.
New cellular system standards such as Enhanced Data-rates for Global Evolution (EDGE) require that the transmitted signal be highly linear from the outset. EDGE imposes stringent requirements with respect to a coupling of power into off channel frequencies and to a magnitude of an error vector (EVM), and does not provide for an initialization period during which a communication device with a Cartesian feedback transmitter may run open loop and train with dedicated training signals. As a result, in order to meet the EDGE requirements, the communication device must run open loop with a highly linear bias level and must operate the RF amplifying element with significant power back off below a gain compression point. However, this reintroduces the issue of low efficiency that is overcome by operating closer to compression and using a distortion compensation technique.
Therefore, a need exists for a method and apparatus that provides the higher efficiency of a non-linear bias level combined with negative feedback correction and that meets the stringent off channel and error vector magnitude (EVM) requirements of cellular standards that do not allow for the initialization period required for a Cartesian feedback transmitter.